In recent years, various processed products formed from rubber or elastomers have been used in a wide variety of fields. These processed products use, as the starting materials, rubber compositions which are comprised mainly of rubber or elastomers and secondarily contain various types of fillers, additives, coloring agents, reinforcing agents, etc. so as to both secure the inherent properties of the rubber or elastomer and also impart or improve various properties required for the specific applications. In general, a rubber or elastomer alone cannot give the required reinforcement, and therefore, carbon black is compounded or formulated as a filler. However, silica etc. is sometimes compounded, together with the carbon black or in place of the carbon black, when the rubber product required is not black. For example, in the applications such as footwear, conveyor belt covers, rolls, hoses, cables, rubberized fabrics, grips, various colors are required, depending upon the applications.
However, rubber compositions containing silica, suffer from phenomena such as the increase in the viscosity, the scorching, the delayed vulcanization, when unvulcanized, which cause inferior processability or poor productivity. These unique phenomena in rubber compositions containing silica are believed to be due to the silanol groups present on the silica surface. That is, the increase in the viscosity is due to the cohesion of the silanol groups, which forms structural body in the rubber composition, whereby the viscosity is increased, the vulcanization accelerator etc. are adsorbed by the polarity of the silanol groups and, as a result, the vulcanization reaction is delayed, the performance in mixing is decreased due to the insufficient compatibility with the nonpolar rubber and, as a result, the processability of the unvulcanized rubber composition is decreased.
Further, silane coupling agents are often used together with rubber compositions containing silica. However, silanol groups are also present in the internal cavities of the silica particles. These silanol groups are reacted with the silane coupling agent to cause a loss of the silane coupling agent and, as a result, the reinforcing effect is decreased. Therefore, there was the problem that a large amount of silane coupling agent had to be compounded.
Various methods have been proposed in the past to solve these problems unique to rubber compositions containing silica. For example, methods have been proposed of adding a polar substance such as diethylene glycol or a fatty acid (see Rubber Industry Handbook, 4th edition; Tire Technology International 1995, p. 107 to 108; JP-A-6-248116, etc.). However, according to these methods, when the polar compounding agent such as diethylene glycol is added to the rubber composition, the adsorbing phenomenon of polar compounding agents such as vulcanization accelerator is prevented to a certain extent, but they cannot completely prevent it. They cannot prevent substances chemically bonding with silica such as the silane coupling agent from bonding in the internal cavities of the silica and, therefore, cannot be said to be methods which can be fully applied in practice to completely resolve all problems.
Further, elimination of these problems inherent to rubber compositions containing silica and the realization of excellent properties in such rubber compositions depending upon the applications of the final products have been sought. For example, in rubber compositions used for the soles of footwear or portions of boots etc. exposed to harsh wear, a superior abrasion resistance of the vulcanizate is sought. Further, in rubber covers of conveyor belts, since abrasion occurs due to the carrying of the transported loads and cracks occur due to the flexing at the pulley portions, a superior abrasion resistance and crack resistance are sought. In rubber compositions used for rolls, due to the abrasion caused by carrying the transported iron metal, paper, etc. and the load due to the transported products, a superior abrasion resistance and bondability with iron metal of the vulcanizate are sought. In rubber compositions used for hoses, since the hoses expand due to the water pressure, oil pressure, etc. and the hoses contact each other or other objects, a superior modulus and abrasion resistance are sought. In rubber compositions used for coverings of cables, considering the lifetime of the products, a superior abrasion resistance is sought. Further, in rubber compositions used for rubberized fabric, in recent years abrasion resistance has been increasingly demanded even for rubberized clothing such as boat parkers. Further, even in the grips attached to the gripping portions of equipment or parts such as golf clubs, bicycles, motorcycles, agricultural machinery, due to the abrasion which occurs each time the user or worker grips them, abrasion resistance is sought in the rubber compositions for the grips as well.